The present invention relates to a method of forming a resist pattern by projection exposure employed in a process of fabricating semiconductor devices, and in particular to photomasks used in such a method.
In optical lithography by projection exposure, various methods have been proposed which can form fine resist patterns to cope with the increase of the packing density of the semiconductor devices. One technique drawing attention is the phase-shifting mask method.
The phase-shifting mask method is disclosed in Japanese Patent Kokoku Publication No. S62-59296.
In the phase-shifting mask method, a phase-shifter in the form of a transparent thin film which shifts the phase of light passing therethrough is disposed partially over a photomask. At the edge of the phase-shifter (the boundary between the area covered by the phase-shifter and the area not covered), the light which passed through the phase-shifter and the light which did not pass through the phase-shifter are 180.degree. out of phase from each other, interfere with each other, and lower the light intensity on the wafer, so that the part of the resist along the edge of the phase-shifter is effectively "unexposed" to the light. By the use of the phase-shifter, the resolution of the projection exposure method is improved.
It is however necessary to provide a measure to eliminate the undesired lines of the resist pattern corresponding to edges of the phase shifter whose images are not desired to be transferred. One measure is to use two photomasks, a first one having the phase shifter, and a second one having an opaque pattern of a transmission mask uncovering the undesired part of the edges of the phase shifter, so that the part of the resist corresponding to the undesired part of the phase shifter edge is exposed during the exposure using the second photomask. This method has shortcomings in that the number of exposure steps as well as the number of photomasks are increased.
Another problem with the prior art phase shifter method is that resist pattern having lines with different width could not obtained.